Tweaking the About:config page is a great way to make Firefox look and behave the way you want it to, but if you're not too comfortable digging around in a table full of options, or you'd rather have a proper interface for them, try Gui:Config.

Gui:Config is an add-on for Firefox that gives all of the About:config options proper check-boxes and drop-down menus that indicate what each option does and what happens when you toggle it. It's ideal for people who want to start tweaking some of the settings available, but aren't sure what setting an item to O versus 1 actually does. Gui:Config is free, and works anywhere Firefox does.